MEMS Microphones are extremely sensitive pressure sensors. At the lower end of the dynamic range, a MEMS microphone can detect pressure fluctuations of 1/1000 Pa or even less. During manufacturing, assembly, and use, a MEMS microphone may also be subjected to static or dynamic pressure pulses of up to at least one bar (100000 Pa). For example, some individuals direct pressurized air at the devices in order to clean the devices, although this practice is typically not recommended. The large dynamic range (1/1000 Pa to 1000000 Pa) is typically accommodated by incorporating dedicated overtravel stop structures (OTS) that limit the movement of the membrane under extreme overload conditions.
The OTS protects the membrane and also prevents shorting between the membrane and an adjacent electrode which is used to detect deflection of the membrane. Contact between the membrane and the electrode can create a short and presents the potential for destruction of the electronics, or the MEMS structure itself. In some approaches, electronic protection is provided by series resistors or insulating layers on top of the OTS. The use of series resistors requires careful design of the electronics, and the use of insulating layers increases the complexity/cost of the device significantly and may even be impossible due to process constraints. In addition, an insulating layer on top of the OTS is not an ideal solution as long as the membrane and the OTS are at different electrical potentials. In this case, electrostatic forces can decrease the pull-in voltage and/or provide sufficient force to keep the membrane stuck to the electrode, typically the back plate, after contact. Additional circuitry may be required to detect such failures and switch off the system to allow the membrane to release from electrode.
Of course, even if protection from overtravel in the direction of the electrode (back plate) is provided, the device can still be damaged by overtravel away toward the substrate. While various attempts have been made to provide for OTS in the direction of the substrate, the known approaches require increased fabrication costs or incur other disadvantages. In devices which use the substrate above which a membrane is suspended as an OTS, a back cavity is formed in the substrate and the edge of the cavity functions as an OTS. This approach does not require additional manufacturing steps. However, the cavity is formed from the back side of the device while the membrane is formed from the front side of the device. Consequently, the mask used to form the cavity must be aligned with features on the opposite side of the device. Aligning backside features to front side features introduces error. Moreover, the process used to form the back side cavity, typically a High Rate Etch (DRIE) process, is less precise than other processes.
Another embodiment of this approach includes a main backside cavity that is only etched partially through the substrate. Inside this large cavity, a second cavity is formed to extend completely through the substrate. While this can reduce variations resulting from the etch processes involved, it still requires front side-to-back side alignment.
Because of the inherent inaccuracies in backside formation of OTS, devices incorporating the above described OTS must be designed to accommodate the described errors. Thus, the size of the devices is increased in order to ensure sufficient overlap between the membrane and the substrate portion providing the OTS. This increases material costs and introduces wasted space in the device. Moreover, even in an optimized production process, the variability of the overlap in the above described approaches creates variable robustness and also a variable capacitive load as well as a risk of electrical pull-in to the substrate. All of these shortcomings must be accommodated in the design of the device.
The shortcomings above were addressed by a system described in U.S. Pat. No. 8,625,823 which issued on Jan. 7, 2014. In the '823 Patent, existing layers of a device are modified to create an OTS that does not have the disadvantages of the previous approaches while not incurring additional processing costs. Specifically, an OTS portion of the back plate is connected directly to the membrane and insulated from the rest of the back plate by a trench formed by etching. The OTS portion moves together with the movable membrane and contacts an unreleased portion of the membrane layer which is supported by the back plate to limit travel toward the cavity. This approach greatly increases the robustness of the device. There may still be situations, however, where even greater robustness is needed. For example, because the OTS structures must be electrically isolated, robustness is compromised due to the limited number of OTS which can be placed around the membrane. Thus, the approach of the '823 Patent is inherently inferior to an OTS which extends completely about the membrane.
In view of the foregoing, it would be advantageous to provide an accurately positioned OTS. It would be advantageous if the OTS could be incorporated using known MEMS processes. It would be further advantageous if the OTS could be easily adapted to provide increased/decreased robustness for particular applications.